This invention relates to a cooling process for thermoplastic resin film which cools the out surface reverse to a cooling drum of a thermoplastic resin film extruded onto a cooling drum by air flown out through air nozzles in addition to the cooling by the cooling drum, and an apparatus therefor.
In general, a melted thermoplastic resin is extruded onto a cooling drum in a thin film through a die, and cooled by the cooling drum to produce a thermoplastic resin film. Besides, it is known that a surface reverse to the cooling drum of the thermoplastic resin film (a surface not touching the cooling drum) is blown to be cooled by an auxiliary cooling apparatus in order to cool the surface effectively.
Such an auxiliary cooling apparatus, for example, has at least three air supply openings for blowing clean air, and the length of respective air supply openings in a right angle direction to the transporting direction of the thermoplastic resin film is longer than the width of the film. Each air supply opening has an air blowing angle wherein the extended straight line in the blowing direction conforms to a tangent of the cooling drum, and has exhaust grooves between the air supply openings (Japanese Patent
Besides, there is a pinning apparatus to press an extruded thermoplastic resin film on a cooling drum which also has a construction to blow air toward the thermoplastic resin film although it is not the auxiliary cooling apparatus. In the pinning apparatus, a plate having very small holes, a porous material or a very fine mesh is attached to the end of the blow-off pipes (Japanese Patent KOKAI NO. 61-239928), and the other means are also disclosed in Japanese Patent KOKAI Nos. 61-135725, 48 4564 and the like.
In the above conventional cooling apparatuses, since the end portion of the air nozzle is at a low temperature, low molecular weight substances sublimated from the thermoplastic resin are condensed on the end surface of the air nozzle.
Moreover, the escape of the blown air toward the die can not be prevented completely. Therefore, the low molecular weight substances sublimated from the thermoplastic resin are condensed in a powder state by cooling with blown air, and the powder drops on the thermoplastic resin film. The blown air impinges on the thermoplastic resin film in a melting state resulting to affect the flatness and thickness adversely.
The pinning apparatus to press a thermoplastic resin film on a cooling drum is almost the same as the cooling apparatus in view of blowing air on a thermoplastic resin film, but since the object of the pinning apparatus is only in pressing the thermoplastic resin film on a cooling drum, the cooling effect on the extruded film is little.